Best software for designing on Ipad

I am a high school teacher who teachers tech ed and am looking to incorporate 3D printing into some of my classes. All the students were each given their own Ipad's this year so I am trying to use with 3d printer them instead of having to spend any extra money on computers and Solidworks for my classroom.

What would you recommend that I use to have the students to design their projects in on their Ipad? I want something that will be on the easier side for them to learn the basics of how to use but still have the technical capabilities to allow them to make something technical and complex?
I've seems their are quite a few options, but I don't have the time to try them all. Any input would be appreciated!

Hmm, my first thought is to simply ask the students to each try a different one out and see who finds the best.

I pretty much just use openscad, and while i do have a very good version that runs on android - you need a bluetooth keyboard to go with it otherwise the onscreen keyboard just obscures everything.

But yep, I'd make it a class project - get them to track different packages down and try them out. Person who produces most useful and printable 'thing' wins a prize :-)

I would start them with OpenScad. A 10 line program in OpenScad can make some really cool shapes. And that will work on their iPads. And it might get some of the kids interested in computer programming. OpenScad is a very simple computer programming language. I would also suggest test questions such as showing a shape and say "Write a 4 or 5 line OpenScad program that will generate this shape."

well there is an online version that should work through even an ipad browser. No idea if there are any versions for apple os. I've got 2 for android but only one is of any real use, but like I said it doesn't really work well with the onscreen keyboard. And balancing the tablet and wireless keyboard on my lap is awkward.
And if I'm going to sit down and do it - might as well sit down at a proper computer.

There must be drag and drop apps for ipads, yeah it's apple so you'll probably have to pay something for them (yet another advantage of android ) - but must exist.

Read this - http://forum.openscad.org/OpenSCAD-for-iOS-td21686.html

This is your best bet: https://openjscad.org/

But I don't really think it's that suitable for ipads.
This looks promising: https://www.gravitysketch.com/app/
Have to see if there's an android version.